Soil salinity, a critical environmental stressor, substantially impacts plant growth and productivity. It induces osmotic stress, disrupts ion homeostasis, and triggers the excessive production of reactive oxygen species (ROS), which can lead to oxidative damage within plant cells. To counteract these detrimental effects, plants have evolved sophisticated defense mechanisms, one of which involves the production of secondary metabolites (SMs). These SMs function as biostimulants that bolster antioxidative defenses and modulate signal transduction pathways, thus enhancing the plant's tolerance to salt stress. Recent evidence reveals SMs like sulforaphane (glucosinolate-derived) uniquely stabilize redox cofactors and reprogram stress-responsive miRNAs. Furthermore, they influence key signaling cascades, such as the mitogen-activated protein kinase (MAPK) pathway and various hormone-regulated pathways, which are instrumental in orchestrating adaptive responses to saline conditions. The regulation of SMs biosynthesis under salt stress is mediated by transcription factors like MYB, WRKY, and bHLH, which are essential for activating the genes involved in these metabolic pathways. Elucidating the intricate mechanisms by which SMs operate as biostimulants not only advances our understanding of plant stress responses but also paves the way for developing sustainable agricultural practices aimed at improving crop resilience in saline environments. This knowledge is instrumental for cultivating crops that can thrive under challenging soil conditions, ultimately contributing to global food security.

Climate change events significantly impact the food production chain by damaging crops in their most fragile phenological states. Furthermore, increasing human population and excess food waste present agricultural systems with the challenge of closing the yield gap and securing food demands in the future as well as protect the soil health and biodiversity. Biostimulants are a novel alternative in agriculture that can effectively use inputs, enhance crop resilience to abiotic stresses and improve food quality. Additionally, biostimulants offer a promising and eco-friendly solution for reducing the use of chemical fertilizers, as they have the potential to increase crop nutrient use efficiency and yield. Because of their effects on plant growth, a wide range of products can be marketed as biostimulants. Presented in this review is an overview of recent literature on the use of plant growth-promoting microbes and microalgae-derived extracts obtained from either waste streams or recycled substrates. Starting from their source material, extraction technologies and application modalities, a view of their factors shaping the composition and activity of biostimulants is provided to elucidate a mechanistic model of action which leads to increased stress resilience in crops. This work further sets out to understand if the biostimulants can be used to transform waste into a valuable product that can accelerate the transition to sustainable agriculture.This article is part of the theme issue 'Crops under stress: can we mitigate the impacts of climate change on agriculture and launch the 'Resilience Revolution'?'.

Environmental stresses, particularly drought and salinity, significantly impair plant growth and productivity. This study explores the novel synergistic interaction between biochar and arbuscular mycorrhizal fungi (AMF) in enhancing the resilience of sweet pepper plants subjected to the individual or combined stresses of drought and salinity. The impact of these biostimulants on growth parameters, photosynthetic efficiency, and biochemical traits was assessed. Sweet pepper plants were subjected to drought stress (35 and 75% of field capacity (FC)), salinity (0 and 150 mM NaCl), and their combined effects (150 mM NaCl +35% of FC), with treatments including biochar (2.5 g/kg soil), AMF, and their combination. Under drought stress, the dual application of biochar and AMF notably improved plant growth indicators such as shoot fresh weight, shoot height, and number of leaves by 50, 14, and 3%, respectively compared to the control plants. Under drought and salinity combined, this combination also enhanced photosynthetic pigments content by 144% for Chl a, 316% for Chl b, 212% for Chl T and 302% for carotenoids content respectively compared to the control plants. Additionally, AMF and Biochar combined reduced the oxidative effect of malondialdehyde (MDA) by 37% and hydrogen peroxide (H2O2) by 43%, indicating a reduction in oxidative damage. Furthermore, a significant increase in antioxidant enzyme activities was observed, with peroxidase activity (POX) rising by 33% and polyphenol oxidase activity (PPO) increasing by 212%, indicating enhanced stress tolerance. This study underscores the efficacy of using biochar and AMF together to bolster sweet pepper plant resilience, offering a viable strategy for improving plant performance under challenging environmental conditions.

Global climate change has significantly reduced the yield of many crops due to various abiotic stressors. These stressors include water-related issues such as drought and flooding, thermal changes like extremely low and high temperatures, salinity, and adverse soil pH conditions including alkalinity and acidity. Biostimulants have emerged as promising and effective tools for mitigating the damage caused by these abiotic stressors in plants, ultimately enhancing both the quantity and quality of crops. Biostimulants are naturally derived substances that include humic acid, protein hydrolysates, nitrogenous compounds, seaweed extracts, beneficial bacteria, and molds. Even at low concentrations, biostimulants play a critical role in activating important plant enzymes, inducing antioxidant defenses, improving water relations and photosynthetic activity, stimulating hormone-like activities (particularly auxins, gibberellins, and cytokinins), and modulating root system development. This review discusses the physiological effects of microbial biostimulants on the quality and productivity of fruit crops, as well as their experimental applications.

Currently, extreme weather events caused by climate change, such as heat waves, drought, frost, and heavy precipitation, have become a threat to agriculture by detrimentally affecting plant productivity and quality. The overuse of synthetic fertilizers is another major concern damaging the soil quality and water and air quality. In this regard, biostimulants could be a promising and potent solution to address these environmental concerns and meet the need for developing sustainable and green modern agriculture. Biostimulants that are primarily composed of natural substances and/or microorganisms can be broadly divided into non-microbial and microbial categories. In this review, the applications of the main types of biostimulants to plant growth and development are discussed, and the possible associated mechanisms of action are described as well. Furthermore, the current status and challenges relating to commercialization and large-scale implementation under changing climate conditions are covered. Overall, this review article could offer insights and knowledge of biostimulants' uses in agriculture for both academia and industrial sectors.